The design of this database is hidden (for example) but if you access it via the Notes client it's not hard to find out the fieldnames and values thru a toolbar button or other code. For instance, here's a toolbar button formula:

There are only two ways to securely keep a field value secret: encrypt the field, or store it elsewhere, not in the document, in a place to which the user you want to hide it from has no access. If it's in another Notes document, you can pull it in by @DbLookup, for instance.